1. Field of the Invention
The present invention relates to a developing device comprising plural developing rollers to supply toner to an image carrier for developing a latent image, and used in, for instance, copying machines.
2. Description of the Related Art
In the field of electro-photography, printing speed of printers and copying machines have become extremely fast recently.
However, with the increase in the printing speed of an electro-photographic printer, the rotary speed of a photosensitive drum serving as an image carrier, that is, a process speed becomes fast accordingly. Because amount of development per unit time of a developing device (amount of toner to be adhered to the photosensitive drum) becomes less, the printing capacity drops basically. In other words, the density of image to be printed on a paper drops.
In addition, from the viewpoint of the photosensitive drum, when the rotary speed of a photosensitive drum becomes fast, it is inevitable to increase the diameter of the photosensitive drum for sensitivity, etc. When the diameter of the photosensitive drum becomes large, image quality tends to be deteriorated. When the print speed becomes fast, various problems including drop of image density, deterioration of image quality, etc. will result and it becomes necessary to solve these problems. As a means for solving these problems, there is a so-called plural roller developing device equipped with plural developing rollers (developer carriers) in small diameter in one developing device.
In the case of such a plural developing roller type developing device, it is possible to extract superior performance of any of image density, fogging characteristic, document reproducibility and life characteristic exceptionally higher than a single developing roller type developing device. However, there are also some problem points.
What is most important among the problems is that developing bias voltage is supplied to plural magnetic rollers serving as plural developing rollers from the same transformer and that an all over developing or a carrier-adhering on a white background generating at the time of starting and stopping the developing device caused therefrom.
These phenomena will be described below.
FIG. 1 is a diagram showing the relationship between a development contrast/a white background contrast versus all over developing/a carrier-adhering on a white background (in the case of reversal development). The development contrast referred to here is a difference between developing bias voltage (hereinafter called VB) and residual potential (hereinafter called Ver) and concerned in an image density and an image is developed according to this potential difference. On the other hand, the white background contrast is a difference between VB and surface potential (hereinafter called VO) of a photosensitive drum and is concerned in the carrier-adhering on a white background and fogging.
As shown in FIG. 1, the magnitude of the development contrast is equal to that of the image density. That is, the more the development contrast becomes large, the more thick development, that is, a so-called all over developing is made. On the other hand, regarding the contrast on a white background, when it is large, the carrier-adhering increases and when it is small, the fogging level becomes worse. Such phenomena are generated from the fact that VO, VB, Ver and toner are all the same polarity and carrier particle only is reverse polarity.
Next, the all over developing phenomenon or the carrier-adhering on a white background that is generated from the supply of developing bias voltage to plural magnetic rollers from the same transformer when starting and stopping a developing device will be described.
First, the operation at the time of the normal starting will be described referring to a timing chart shown in FIG. 2. First, the photosensitive drum drive motor that rotates the photosensitive drum is turned on and reverse bias voltage is output. The reason for outputting reversal developing bias voltage here is that a potential difference between the photosensitive drum and a sleeve of the developing roller must had been generated when the not charged area on the photosensitive drum passes the front surface of the developing rollers. Because, the fogging phenomenon comes out on the photosensitive drum since the developing rollers are rotating.
In other words, if the potential on the photosensitive drum was zero and the potential on the developing rollers was zero, the white background contract will become zero and the considerably remarkable fogging phenomenon would be produced. In this case, reverse bias voltage is voltage of reverse polarity to potential on the photosensitive drum and regular bias voltage is voltage of the same polarity to potential of the photosensitive drum. This reverse bias voltage is generally used for the reversal development.
Then, the developer drive motor is turned on in succession to operate the developing device. Thereafter, charging voltage is output to the photosensitive drum and lastly, regular developing bias voltage is output. In the case of a single roller developing device, there is no problem at this timing. However, in the case of a plural roller developing device, various problems will be generated depending on the output timing of regular developing bias voltage.
Kinds of defective phenomena differ depending on the turn-on timing of developing bias voltage when the charged area on the photosensitive drum has reached the developing roller at the upper stream side in the developing device and when the charged area has reached the developing roller at the downstream as shown in FIGS. 3A and 3B. Whenever developing bias voltage is supplied from the same transformer, the above-mentioned problem will be generated when a plural roller developing device is started. That is, there is no proper timing for applying developing bias voltage that is adequate for generating no defects. Needless to say, it is possible to solve the above-mentioned problems if a developing bias voltage transformer was provided for each developing roller; however, cost will increase by two times and the actual adoption is not realistic.
Next, the timing at the of normal stopping will be described referring to FIG. 4. That is, after detecting the trigger of completing the print, the charging voltage output is first turned off and in succession, regular developing bias voltage is switched to reversal developing bias voltage. The reason for why developing bias voltage is switched to reverse bias voltage instead of turning-off at this time is the same as that at the time of starting. That is, because the developing rollers are rotating, if a difference between potential of the photosensitive drum and bias voltage of the developing rollers is zero, the fogging phenomenon is presented on the photosensitive drum. So, when developing bias voltage is made to reverse polarity, the fogging phenomenon is not generated. Use of reverse bias voltage here is a practice of common sense.
In succession, the developing device is stopped (the developer drive motor is turned off), the photosensitive drum is then stopped (the drum drive motor is turned off) and lastly, reversal developing bias voltage is turned off. In the case of a single roller developing device, no problem will be generated at this timing; however, in the case of a plural roller developing device, defects can be generated depending on the timing for switching regular developing bias voltage to reversal developing bias voltage.
That is, a kind of defective phenomenon differs between the timing for switching the regular developing bias voltage to reversal developing bias voltage when the not-charged area on the photosensitive drum reached the developing roller at the upper stream and that when it reached the developing roller at the downstream.
As shown in FIGS. 5A and 5B, when the developing bias voltage is supplied from the same transformer, the all over developing or the carrier-adhering on a white background will be generated whenever stopping a developer equipped with plural rollers. That is, there is no adequate timing for turning off the developing bias voltage so that no defect is produced.
Thus, the above-mentioned problems will be generated on prior art developing devices equipped with plural developing rollers when starting and stopping. Further, if the above-mentioned problems are generated, problems shown below will result.
First, as regards the all over development, as all over developing is made excessively at every printing, such problems as increased toner consumption, decreased amount of recovered toner, increased toner scattering, short-life of cleaning mechanism (blade, fur brush, etc.) of the photosensitive drum itself and peripheral equipment (a transferring belt, etc.) in contact with the photosensitive drum will result.
On the other hand, as regards the carrier-adherence on a white background, it is also produced at every printing, such problems as damage of the photosensitive drum, short-life of the cleaning mechanism of the photosensitive drum itself and peripheral equipment connected to the photosensitive drum, drop of developing efficiency of the developer and loss of the developing function (generation of all over blurring, blade traces) due to decreased amount of toner, abnormal output value of an automatic toner sensor related to the input of toner, etc. will result. Thus, even when either one of the all over developing and the carrier-adhering on a white background is generated, the life of the developing device and copying machine equipped with this developing device will be shortened.
So far, apparatuses using a reversal developing device for the above-mentioned problems that are generated in the reversal development are a printer, facsimile machine, low price digital data using copying machine, etc. and are set for a short life. It is therefore the actual circumstances that even where there are the above-mentioned problems, no countermeasures were especially taken and products were manufactured without solving the problems.
However, high speed and long life digital data using copying machines have been promoted in recent years and it has been necessitated to solve the above-mentioned problems.
So, one plan for solving the above-mentioned problems when a high-speed digital data using copying machine using a developer equipped with plural developing rollers was opened to the public (Japanese Laid-open Publication No. 5735/1995). However, this plan relieves the above-mentioned problems and has no effect to prevent them completely.
That is, in the case of the image forming apparatus disclosed in Japanese Laid-open Publication No. 5735/1995, the photosensitive drum is started, the main charger is turned on and the developing bias voltage is switched to regular bias voltage -600V when the leading edge of the charged area o the photosensitive drum passes the position facing the second developing roller in the state with the reverse developing bias voltage +50V applied to the first and second developing rollers. Then, before the leading edge of an image portion on the photosensitive drum reaches the position facing the first developing roller, the operation of the developing device is started.
However, in the case of the image forming apparatus disclosed in Japanese Laid-open Publication No. 5735/1995, until the application of regular developing bias voltage starts, the charged area on the photosensitive drum passes the position facing the first developing roller with no regular development voltage applied, carrier particles on the first developing roller tends to move to the photosensitive drum side by the electric field produced by the charging potential on the photosensitive drum and such a problem that carrier particles to the photosensitive drum tends to be generated.